add .rtf to download filetypes #15975

fixes https://github.com/matomo-org/matomo/issues/14579 (it's easier to just add it instead of keeping the issue open).

I can't think of any false-positives ending with .rtf so it should be fine.

Let's see if the github action also works for me :)

build js

Yay! :tada:

CSS in the UI test is broken, but I doubt this change caused it.